16.6 Biological control of arthropod pests

We have just described a range of arthropod diseases that can govern the
development of arthropod populations in nature. They kill specific arthropods
and in so doing produce an increased supply of infective agents that can kill
more of the same arthropod. Put in those terms, and against the background of
our need to produce crops that are subject to attack by numerous arthropod
pests, and suffer diseases that are transmitted by other such pests, it is not
at all surprising that great research effort is devoted to attempting to harness
these diseases into biocontrol agents that can be used to
attack those arthropods that we view as pests. Biological control has been
defined as the practice by which the undesirable effects of a pest organism are
reduced through the agency of another organism that is not the host, pest or
pathogen, or man.

Synthetic chemical insecticides and acaricides
have been the most important element of arthropod control programmes since the
discovery of the insecticidal properties of DDT in the late 1930s. They remain
the most important element today because they each:

have a broad spectrum of activity that makes them
effective against several different types of arthropod;

are highly efficient;

are cheap and easy to produce;

are persistent, and continue to kill pests
throughout the season or the field life of the crop.

Chemical pesticides have served us well in the latter half of the 20th
century by reducing the economic damage caused by arthropod pests and by
reducing the incidence of many of the diseases of plants and animals that are
transmitted by arthropods because of this combination of properties.

But these same properties have also created the problems widely associated
with use of chemical pesticides. Because they are highly efficient and cheap and
easy to produce they were rapidly brought into heavy and widespread use.
Their broad spectrum of activity lead to their use throughout the world and in
all circumstances in which control of arthropod pests and/or vectors of human
and animal diseases was an issue, even if only minor. Their persistence combined
with this extensive use resulted in many of these pesticides becoming unnatural
components of the natural environment, contaminating land and food and
water supplies. The varied results included:

development of resistance in many pest populations;

emergence of new pests as old pests were removed;

elimination of natural enemies of pests;

disruption of natural ecosystems as non-pest organisms were also
killed;

accumulation of pesticides through food chains to
adversely affect bird and mammalian predators.

All of these negatives directed attention towards increased emphasis on
alternative control agents and strategies, which include:

biological control (biocontrol) agents;

environmental management;

use of pheromones to control pest mating;

genetic modification to produce arthropod-resistant
plants;

combination of these tactics into integrated pest management
(IPM) programmes.

Naturally occurring pathogens of arthropods are good
candidates as biological control agents and many species are already employed,
at least on a small scale, to control arthropod pests in glasshouse crops,
orchards, ornamentals, turf and lawn grasses, stored products, and forestry, and
for moderation of vectors of animal and human diseases (see review by Lacey
et al., 2001). There are several theoretical advantages of using
microbial biocontrol agents although attempts to harness their
potential so far have had comparatively minor commercial success. Among the
undoubted advantages of biocontrol are:

efficiency and potentially high specificity;

natural

safety for humans and other non-target organisms;

reduced chemical pesticide use and consequent reduction of residues in
food and the environment;

preservation of natural enemies of the pest;

maintenance of biodiversity in managed ecosystems.

But there are some negative aspects of biocontrol:

biocontrol agents can be costly and difficult to produce in quantity;

they can have short shelf life;

the pest must be present before the pathogen can be usefully applied (so
prophylactic, or preventative, treatment is difficult).

The use of entomogenous fungi for biological control has been disadvantaged
by the need for high humidity (80% and above) during the prolonged period
required for fungal spores to germinate and then penetrate the surface of
insects because, unlike bacteria, these fungi attack insects through their
cuticle, not their digestive tracts. To try to overcome this problem researchers
have developed oil-based and other formulations of fungal spores for use in
biological control. Verticillium lecanii (Lecanicillium muscarium)
is an effective biocontrol of Myzus persicae and other aphids on
chrysanthemums because the crop is grown in a glasshouse in which humidity can
be controlled. Furthermore, the crop is ‘blacked out’ with polythene sheeting
from mid-afternoon until morning during the summer to control the initiation of
flowering (it’s a ‘short-day’ flowering cycle) and this helps to create a high
humidity around the plant in which the fungal spores can germinate and infect
the aphids. A single spray of spores given just before ‘black-out’ gives
satisfactory control of the aphid within 2-3 weeks. Spores of Verticillium
lecanii can be used to control whitefly as well as aphids.

It is much more difficult to use entomogenous fungi for biological control in
the field, although there is promise for biocontrol of pests that have an
aquatic stage. Of importance is the fact that recent research has improved the
prospect of using insect fungal pathogens for control of diseases such as
malaria that depend on insect vectors (Blanford et al., 2005; Scholte
et al., 2005; Thomas & Read, 2007). Among 85 genera of entomopathogenic
fungi only six species are commercially available for field application:
Aeschersorzia aleyrodis, Beauveria bassiana, B. brongniarti,
Metarhizium anisopliae, Paecilomyces fumosoroseus, and
Verticillium lecanii (Hussain et al., 2012).

So, although several fungi are in use or have promise, the most widely
used microbial control agent of arthropod pests now is the bacterium
Bacillus thuringiensis (Lacey et al., 2015). This is used as
the model for biocontrol mechanisms and advances in understanding its
molecular biology, mode of action, and resistance management contribute to
development of fungal control agents (mycoinsecticides),
especially insights into their pathogenic process, enzymes involved with the
penetration of the host cuticle and the role of insecticidal fungal toxins
(Charnley, 2003). Transgenic plants expressing endotoxin genes
from Bacillus thuringiensis have been generated to protect crops
(tobacco, cotton and maize) against attack from pests. It is possible that
the same technology could be applied to toxins produced by entomogenous
fungi.

Increased use of microbial control agents as alternatives to broad-spectrum
chemical pesticides depends on: